1. Field of the Invention
The present invention relates to a technology for removing nonlinear distortion that occurs in digital transmission such as satellite broadcasting, terrestrial broadcasting and cable television broadcasting.
2. Discussion of the Related Art
In recent years the digitization of TV broadcasts has rapidly progressed in the cable, satellite and terrestrial media in Japan as well as in Western countries. In Japan, regular BS digital broadcasting started in December, 2000. In addition, regular terrestrial digital broadcasting is scheduled to start in metropolitan areas by 2003.
In BS broadcasting the transmission distance reaches over several tens of thousands of kilometers. Therefore, an amplifier in a transponder within a broadcast satellite has little back-off and operates in the area wherein amplification factor is high. Therefore, radio waves emitted from the transmission station receive nonlinear distortion and are transmitted to a reception antenna of each home from the broadcast satellite. In addition, an amplifier is mounted in a receiver for amplifying the received signal regardless of the media, such as satellite, terrestrial or cable, so that the received signal, of which amplitude is large, is affected by the nonlinear characteristics of the amplifier.
On the other hand, a reproduction head (hereinafter referred to as MR head) utilizing a magnetic resistance effect is used in a magnetic recording and reproduction apparatus, such as a magnetic disk apparatus (HDD). In a magnetic recording and reproduction apparatus that uses an MR head, the magnetic field-reproduction output conversion characteristics become nonlinear due to a shift of a biased magnetic field or due to the dispersion of the magnetic characteristics of the MR element. Therefore, the reproduction signal waveform receives nonlinear distortion.
The above described nonlinear distortion becomes a major factor that causes the deterioration of the error ratio, and the like. Conventionally, as for a method for compensating for nonlinear distortion, there is a configuration wherein a front end equalization circuit having opposite characteristics to the amplifier is provided in front of the amplifier in, for example, the transponder (Tsuzuku, et al “Advanced Satellite Broadcasting System in the 21 GHz Band, 16 QAM Transmission with Pre-distortion,” technical report of Institute of Television Engineers, BCS94-25 (August 1994)).
In addition, a nonlinear distortion equalization method in a magnetic recording and reproduction apparatus utilizing an MR head is shown in, for example, Japanese unexamined patent publication H9(1997)-7300. FIG. 1 shows, as extracted from the above gazette, a configuration diagram of the entirety of the nonlinear compensation equalizer. This nonlinear compensation equalizer is formed to include an amplitude value converter 1, an FIR filter 2, an equalization error calculator 3 and an LMS algorithm coefficient learning apparatus 4, as shown in FIG. 1.
The amplitude value converter 1 has a multiplier 12a for squaring the input of a reproduction waveform 11, a multiplier 12b for multiplying the reproduction waveform 11 by the output of the multiplier 12a, a coefficient multiplier 14a for multiplying the output 13 of the multiplier 12a by a coefficient value c2, a coefficient multiplier 14b for multiplying the output of the multiplier 12b by a coefficient value c3 and an adder 15 for adding together the output of the coefficient multiplier 14a, the output of the coefficient multiplier 14b and the reproduction waveform 11.
The FIR filter 2 shown in FIG. 1 has a first coefficient multiplier 22a for multiplying a first tap input value 21a by a coefficient value h1, . . . an N-th coefficient multiplier 22n for multiplying an N-th tap input value 21n by a coefficient value hn, a delay element 23a for sequentially delaying the input signal, . . . a delay element 23n, an adder 24 for adding together the outputs of the first coefficient multiplier 22a, . . . the N-th coefficient multiplier 22n. 
The equalization error calculator 3 has a subtracter 31 for calculating the difference between the equalization output outputted from the FIR filter 2 and the equalization target and for outputting the value of the difference as an equalization error 32.
FIG. 2 shows a configuration diagram of the LMS algorithm coefficient learning apparatus 4. This LMS algorithm coefficient learning apparatus 4 is formed of a coefficient learning circuit control part 5, a first coefficient learning circuit 6 and a second coefficient learning circuit 7. The first coefficient learning circuit 6 is a circuit for learning the tap coefficient of the FIR filter 2 of FIG. 1. The second coefficient learning circuit 7 is a circuit for learning the tap coefficient of the amplitude value converter 1 of FIG. 1.
The first coefficient learning circuit 6 has, as a learning circuit of the coefficient value 1h, a multiplier 61a for multiplying the equalization error 32 by the coefficient value 1h, a multiplier 62a for multiplying the step size parameter u by the output of the multiplier 61a, an adder 63a for adding the output of the multiplier 62a to the output of the delay element 64a and a delay element 64a for delaying the output of the adder 63a, which is returned to the adder 63a. In addition, the first coefficient learning circuit 6 has, as a learning circuit of the coefficient value hn, a multiplier 61n for multiplying the equalization error 32 by the coefficient value hn, a multiplier 62n for multiplying the step size parameter u by the output of multiplier 61n, an adder 63n for adding the output of the multiplier 62n to the output of the delay element 64n and a delay element 64n for delaying the output of the adder 63n, which is returned to the adder 63n. 
The second coefficient learning circuit 7 has, as a learning circuit of the coefficient value c2, a multiplier 71a for multiplying the equalization error 32 by the coefficient value c2, a multiplier 72a for multiplying the step size parameter u by the output of the multiplier 71a, an adder 73a for adding the output of the multiplier 72a to the output of the delay element 74a and a delay element 74a for delaying the output of the adder 73a, which is again returned to the adder 73a. In addition, the second coefficient learning circuit 7 has, as a learning circuit of the coefficient value c3, a multiplier 71b for multiplying the equalization error 32 by the coefficient value c3, a multiplier 72b for multiplying the step size parameter u by the output of the multiplier 71b, an adder 73b for adding the output of the multiplier 72b to the output of the delay element 74b and a delay element 74b which delays the output of the adder 73b, and returns to the adder 73b. 
The operation of the nonlinear compensation equalizer of such a configuration is herein described. The amplitude value converter 1 of FIG. 1 has third order function conversion characteristics. The reproduction waveform 11 that has been reproduced by the MR head is given to the multipliers 12a and 12b so as to gain the squared value and the cubed value. The coefficient multiplier 14a multiplies the squared value by the coefficient value c2. The coefficient multiplier 14b multiplies the cubed value by the coefficient value c3. The adder 15 adds together the reproduction waveform 11, which is of the value to the first power, the output of the coefficient multiplier 14a and the output of the coefficient multiplier 14b. In general, when the amplitude value converter 1 has the third order function conversion characteristics, it can sufficiently compensate for distortion due to nonlinearity of the magnetic field-reproduction output conversion characteristics of the MR head, that is to say, waveform distortion in the reproduction signal waveform.
The FIR filter 2 forms a partial response equalizer. This FIR filter 2 carries out a waveform equalization for giving partial response characteristics. The equalization error calculator 3 finds the difference between the equalization output of the FIR filter 2 and the equalization target. The found equalization error 32 is inputted to the LMS algorithm coefficient learning apparatus 4.
The tap input values 21a to 21n inputted, respectively, to the coefficient multipliers 22a to 22n of FIG. 1 are given to the coefficient learning circuit control part 5 of FIG. 2 as a tap input value sequence 41 (h1_in, . . . ,hn_in) of the FIR filter.
The coefficient learning circuit control part 5 outputs the tap input value sequence 41 of the FIR filter as the tap input values h1_in to hn_in, respectively at a synchronized timing of calculation of the equalization error. Then, the first coefficient learning circuit 6 finds the product of the tap input values h1_in to hn_in and the equalization error 32 with respect to each tap and multiplies the step size parameter u for controlling the learning speed and stability, respectively. Then, the first coefficient learning circuit 6 adds this multiplication result to the immediately preceding coefficient value that has been stored by using the delay elements 64a to 64n. The coefficient learning circuit control part 5 generates an FIR filter coefficient update command 42 according to the above result and updates the coefficient values h1 to hn.
Tap input values 13a and 13b to each of the coefficient multipliers 14a and 14b are given to the coefficient learning circuit control part 5 of FIG. 2 as a tap input value sequence 43 in the same manner as in the amplitude value converter 1 of FIG. 1. The LMS algorithm coefficient learning apparatus 4 calculates the coefficient in the same manner as the FIR filter 2 by using the tap input value sequence 43 and the equalization error 32 and respectively updates the coefficient values c2 and c3 of the amplitude value converter 1 by outputting the amplitude value converter coefficient update command 44.
The nonlinear distortion equalization circuit that has been conventionally used in a magnetic recording and reproduction apparatus operates in the above described configuration and compensates waveform distortion in the reproduction signal due to the nonlinearity of the magnetic field-reproduction output conversion characteristics of the MR head.
In the above described nonlinear distortion equalization method, however, nonlinear distortion in the complex signal cannot be compensated for in digital transmissions such as those of BS digital broadcasting. In addition, the signals wherein the phase synchronization of the carrier wave is not established are not dealt with as signals as objects of equalization in the above described nonlinear distortion equalization method.
On the other hand, as a method for compensating for nonlinear distortion in the complex signal, a method has been conceived of providing a front end compensation circuit, having characteristics opposite to those of the amplifier, to the transponder of the transmission end. However, a method of compensating for nonlinear distortion in the complex signal on the reception end has not been conceived.